This invention relates to an ultrafast gating apparatus equipped with a device for gating X-ray images, electron images or optical images being projected onto a stripline by a pulse voltage propagating through said stripline.
A microchannel plate (hereinafter referred to as "MCP") equipped with a stripline has conventionally been used as a device for gating X-ray images being projected onto the stripline by a pulse voltage applied thereto. Such a gating device is typically employed in an X-ray imaging system which, as typically shown in FIGS. 8 and 9, comprises MCP 1 and a gold transmission path 2 that is vapor-deposited on the MCP 1 and which also serves as a conversion surface to convert an X-ray to electrons. A stripline is formed by the transmission path 2, the MCP 1 as a dielectric, and a grounded electrode 3 on the output surface of the MCP 1.
When an X-ray image from an X-ray pinhole system 4 or the like is projected onto the transmission path (conversion surface) 2, the resulting electrons are multiplied with a "shutter time" that is determined by the width of the pulse voltage applied to the transmission path 2 and the gain characteristic of the MCP 1. The multiplied electrons strike a phosphor screen 5, where they are converted to visible light. The visible light image on the phosphor screen 5 is equivalent to the X-ray image on the transmission path 2, and has duration of the above-defined shutter time (or gating time).
The gated X-ray imaging system described above is capable of successively picking up images at an interval defined by the difference in the arrival times of the applied voltage pulse at the adjacent X-ray images on the stripline. The principle of this successive imaging is briefly described below with reference to FIG. 8. Three identical X-ray images 2A, 2B and 2C are aligned on a stripline as a result of projection through three pinholes 4A, 4B and 4C. The respective X-ray images 2A, 2B and 2C are to be gated with the gating time as described above, which is approximately 100 ps. The time difference (interframe time) T.sub.F between resulting images is equivalent to the propagation time for the pulse voltage to propagate between the adjacent X-ray images on the stripline, and is expressed by: ##EQU1## where .epsilon..sub.r, is the relative dielectric constant of the MCP 1, C is the velocity of light in vacuum, and D is the interimage distance. If D=2 cm and .epsilon..sub.r =5, T.sub.F is calculated as 149 ps from equation (1). In other words, if the X-ray images 2A, 2B and 2C are aligned at an interval of 2 cm, images gated with a gating time of approximately 100 ps can be obtained at an interval of 149 ps.
The gated X-ray imaging system described above, however, suffers a transmission loss in the stripline, and therefore the voltage on the output side will inevitably become lower than the pulse voltage on the input side. Stated more specifically, a MCP which has many pores suffers a particularly large transmission loss and its gain characteristic will vary exponentially with applied voltage. Hence, the variation in voltage is substantially reflected in the output images. As shown in FIG. 9C, the voltage V.sub.mcp will decrease toward the output side, producing optical images on the phosphor screen that are darker on the output side of the stripline than on the input side.